An automatic injector is a device that enables intramuscular or subcutaneous administration of a therapeutic agent. An advantage of automatic injectors is that they contain a measured dose of a therapeutic agent in a sealed sterile cartridge. As such, automatic injectors can be used in emergency situations to quickly and simply inject the therapeutic agent without having to measure dosages. Another advantage of automatic injectors is that the administration of the therapeutic agent is accomplished without the user initially seeing the hypodermic needle through which the therapeutic agent is delivered, and without the user having to manually force the needle into the patient. This is particularly advantageous when the therapeutic agent is being self-administered.
In some automatic injectors, the therapeutic agent is stored as a liquid solution which is then injected. However, the long-term storage of a therapeutic agent as a liquid solution has drawbacks. For instance, some therapeutic agents are not stable in solution and thus have a shorter shelf-life than their solid counterparts. To address this concern, automatic injectors have been developed that store the therapeutic agent in solid form and mix the solid therapeutic agent with a liquid immediately prior to injection. Such devices are generally referred to as wet/dry injectors. An example of such an injector is found in U.S. Reissue Pat. No. RE 35,986, entitled “Multiple Chamber Automatic Injector,” the disclosure of which is incorporated herein by reference. These injectors require the user to manually rupture a sealing member between the solid and liquid components and then manually shake the injector body to expedite dissolution of the solid component prior to injection. Unfortunately, steps such as manually shaking the injector increase the time needed to administer a dose of the therapeutic agent, which is undesirable in many emergency medical situations where rapid delivery of the therapeutic agent is needed (e.g., nerve gas and chemical agent poisoning).
Therefore, a need exists for a cost-effective automatic injector that stores a therapeutic agent in solid form, does not require manual premixing by the user, and quickly and effectively automatically mixes and delivers the therapeutic agent in a liquid solution.